Prydwen first appears in the Welsh mythological poem Preiddeu Annwn, or "The Spoils of Annwn," a cryptic text of sixty lines in Middle Welsh preserved in the Book of Taliesin, a manuscript compiled in the early 14th century though the poem itself is far older. Scholars have dated the poem anywhere from the 9th to the 12th century on linguistic grounds, and its authorship is attributed to Taliesin, the semi-legendary bard of the 6th century who became in Welsh tradition a shape-shifter, a prophet, and the repository of all knowledge. The poem is one of the most difficult in medieval Welsh literature, dense with allusion and riddle, but its central narrative is clear enough: Arthur leads an expedition to Annwn, the Welsh Otherworld, a realm that lies beneath the earth or beyond the sea or both at once, inhabited by fairies, demons, ancient gods, and the dead. The goal of the expedition is a magic cauldron, a vessel that would never boil the meat of a coward but would boil quickly if the meat of a brave man were put in it, and that was tended by nine maidens. The poem names several fortresses in Annwn that Arthur's men assault: Caer Siddi, the Fairy Fortress; Caer Goludd, the Fort of Hindrance; Caer Vedwyd, the Fort of Mead-Drunkenness; and others. The refrain that haunts the poem is the same in every stanza: "Three shiploads of Prydwen we went to it; except for seven, none returned."

The refrain tells us two crucial things about Prydwen. First, the ship was large enough to carry three shiploads of warriors, or perhaps the phrase means that Prydwen herself was overloaded to three times her capacity, or that a flotilla of three ships sailed in her name. The ambiguity is characteristic of Welsh poetry, where a single phrase can hold multiple meanings. Second, the voyage was catastrophic. Of all the men who sailed to Annwn aboard Prydwen, only seven came back. The poem does not say how the others died, whether in battle against the hosts of the Otherworld, in shipwreck, or in some supernatural doom, but the scale of the loss is staggering. This is not the triumphant Arthur of the later romances, the king who conquers Rome and holds the Round Table. This is a Welsh warlord who gambled his men on a raid into the land of the dead and lost almost everything.

The poem also tells us something about the nature of Annwn itself. In the First Branch of the Mabinogi, the collection of Welsh prose tales that includes the story of Pwyll, Prince of Dyfed, Annwn is ruled by Arawn, a king who exchanges places and shapes with Pwyll for a year, and the realm is depicted as co-existent with the everyday world, accessible through a mound or a lake. In Preiddeu Annwn, Annwn is more dangerous, a fortress realm that must be stormed, and Prydwen is the vessel that bridges the two worlds. The ship is thus more than transport; she is the means by which the living enter the domain of the dead, a role that connects her to the ferryman Charon of Greek myth and to the ships of the Norse dead that sail beyond the western sea.

Prydwen appears again in the Welsh prose tale Culhwch and Olwen, composed in the 11th or early 12th century and preserved in the White Book of Rhydderch and the Red Book of Hergest. In this wild comic adventure, Culhwch, a prince cursed by his stepmother to marry only the daughter of the giant Ysbaddaden Bencawr, comes to Arthur's court at Celliwig in Cornwall to ask for help. Arthur greets him with extravagant generosity, promising him anything in the world, and then lists the things he will not part with: his ship Prydwen, his mantle, his sword Caledfwlch, his spear Rhongomyniad, his shield Wynebgwrthucher, his dagger Carnwennan, and his wife Gwenhwyfar. The list is a catalogue of Arthur's identity, and Prydwen stands at the head of it, the first thing named, as if the ship were the foundation of all the rest.

In Culhwch and Olwen, Prydwen is put to work in three separate expeditions. First, Arthur goes to sea in her in an attempt to capture the bitch Rhymhi and her two whelps, one of the impossible tasks that Ysbaddaden has set for Culhwch. Then Arthur and a small force sail in Prydwen to Ireland to obtain the cauldron of Diwrnach the Irishman, a vessel similar to the cauldron of Annwn in that it would never boil food for a coward. When Diwrnach treats them to a feast but refuses to hand over the cauldron, Arthur's warrior Llenlleawc the Irishman seizes Caledfwlch, Arthur's sword, and slaughters Diwrnach's entire retinue. Arthur and his men take the cauldron and sail back to Wales in Prydwen. Finally, when the monstrous boar Twrch Trwyth and his seven piglets swim from Ireland to Wales to escape the hunt, Arthur and all the warriors of Britain follow them in Prydwen, pursuing the beast across the Irish Sea in a chase that will end only after the boar has laid waste to half of southern Britain.

The name Prydwen itself is Welsh, compounded of pryd, meaning appearance, shape, or form, and gwen, the feminine form of gwyn, meaning white, fair, or blessed. The most common translation is "Fair Form" or "Blessed Form," though some scholars prefer "White/Fair Face," a meaning that would apply as easily to a painted shield as to a ship. This ambiguity may explain why Geoffrey of Monmouth, the Welsh cleric who wrote the Historia Regum Britanniae around 1136 and transformed Arthur from a Welsh folk hero into the king of all Europe, borrowed the name for Arthur's shield. In Geoffrey's account, Arthur carries a shield called Pridwen, adorned with an image of the Virgin Mary, the Holy Mother of God, to keep her memory always before his eyes in battle. Geoffrey also gives Arthur a sword called Caliburn, forged on the isle of Avalon, and a spear called Ron, but no ship. The repurposing of Prydwen from ship to shield has puzzled scholars for centuries. Some suggest that Geoffrey simply liked the sound of the name and applied it to the most prominent piece of Arthur's equipment that lacked one. Others argue that in early Welsh tradition, Prydwen was a magical object that could take multiple forms, a ship when it sailed and a shield when it fought, a shape-shifting vessel that embodied the Celtic belief in the fluidity of forms. The Liber Landavensis, a 12th-century document, records the place-name messur pritguenn, "the Measure of Prydwen," suggesting that the name was associated with a specific location in Wales, perhaps a harbour or a landing place where the ship was once moored.

In later Welsh folklore, Prydwen was gradually replaced by another name. A 16th-century manuscript records that a sandbank called Caswennan off the coast of Gwynedd in North Wales was "a place hateful to ships, near Bardsey and Llŷn; there Arthur's ship named Gwennan was wrecked." The name Gwennan, meaning "white" or "blessed," is clearly related to Prydwen, and the hydrographer Lewis Morris found in 1742 that the same tradition was still current in that locality. The poet Evan Evans repeated the story in 1764, and the antiquarian Iolo Morganwg, in the early 19th century, listed Gwennan among the ships that "conveyed the saints to Ynys Enlli," the island of Bardsey. Whether Gwennan was an alternative name for Prydwen or a separate vessel in a now-lost tradition, the two names share the same root of whiteness and blessing, the same association with Arthur's voyages to the west.

Prydwen has enjoyed a remarkable afterlife in modern literature and popular culture. In John Masefield's 1928 poem "The Sailing of Hell Race," based on Preiddeu Annwn, Arthur's ship is called Britain, a play on the names Prydwen and Prydain, the Welsh name for Britain. H. Warner Munn's 1939 novel King of the World's Edge sends Arthur across the Atlantic in Prydwen. Susan Cooper's Silver on the Tree, the last of her five Arthurian novels for children, ends with King Arthur sailing into the beyond in his ship Pridwen. Guy Gavriel Kay's Fionavar Tapestry features Prydwen in a quest for a magical cauldron. Heather Dale's 1999 song "The Prydwen Sails Again" returns the ship to the voyage to Caer Siddi. And in the post-apocalyptic videogame Fallout 4, the Brotherhood of Steel operate from a dieselpunk airship named the Prydwen, the name chosen by their leader Arthur Maxson from "a work of historical fiction about a man destined to become a king, and his journey to liberate his people from tyranny and oppression."